Doing stupid things to amuse yourself seems like exactly the opposite of everything Voldemort has ever done. If ever anyone has epitomized ruthless efficiency, he's the guy.

That does not match my model of canon Voldemort, and I assume that MoR pre-Harry Voldemort was very similar.

I expect Voldemort to kill/torture valuable minions because of impulses ("to amuse yourself") even if it would harm his overall goals.